1. Technical Field
Generally, the present invention relates to the encapsulation of semiconductor chips on ceramic modules with a protective cap. More particularly, it relates to an apparatus for and a method using the apparatus for the encapsulation of the ceramic substrate through the use of radio frequency induction heating.
2. Background Art
In the fabrication of a modern computer, one or more semiconductor chips which provide the main logic and memory circuits for the computer are electrically connected to a ceramic substrate. The ceramic substrate is in turn electrically connected to a printed circuit board and several circuit boards in the computer.
Before the ceramic substrate is mounted on the circuit board, it is encapsulated by covering the semiconductor chips on the substrate with a protective cap, generally composed of a metallic material, which is bonded to the periphery of the ceramic substrate. The assembled ceramic substrate and metallic cap combination is called a module or module assembly. In the prior art, the encapsulation process requires large and expensive tooling, as well as a considerable amount of process time and operator attention. To attach a metallic cap to the ceramic substrate, a metallic sealband, typically of tin solder, silver solder or other suitable brazing material, is applied to the periphery of the substrate. The sealband must be flattened by placing a ceramic weight on the sealband and running the substrate-weight combination through a finely controlled furnace. Next, after removing the ceramic weight, a thermal paste is dispensed into the cap. A capping tool then positions the metallic cap onto the substrate. The thermal paste acts as a thermal bridge between the semiconductor chips on the substrate and the cap to aid in heat dissipation through the cap. A capping fixture is then attached around the cap and substrate to apply the necessary pressure for the bonding operation. The substrate-cap-capping fixture combination is subsequently run through another finely controlled conveyor furnace with an inert atmosphere. In the prior art method, the bonding operation is quite slow as both the module assembly and the capping fixture must be heated to the reflow temperature by means of convection heating. After the module is assembled, it undergoes a series of electrical and physical tests to determine whether all components are operating and have been assembled correctly. Occasionally, a module fails testing. The failed module is then disassembled by running through the conveyor furnace and those parts which are fully functional are salvaged for other modules.
The prior art encapsulation process described above is time consuming, expensive, and requires a significant amount of floor space in the semiconductor fabrication line. For example, a single conveyor furnace measures 25 feet long by 3 feet wide, and 5 feet high, requires constant temperature profiling throughout its lifetime. The combined sealband flattening and module bonding processes in such a furnace require an average of 40 minutes for each module. The furnaces, once profiled, remain continuously powered and are continuously purged with nitrogen or another inert gas; over time, both the power and purging gas represent significant operating costs. Further, as floor space on a semiconductor fabrication line is generally figured at $100 per square foot per annum, the size of the tool alone represents a significant cost factor. Adding yet further to the costs of the prior art encapsulation method are the multitude of capping fixtures which must be built to obtain a sufficiently high throughput. The capping fixtures must be specially made for each type of module assembly manufactured in the line.
Finally, in addition to the cost problems associated with the prior art process discussed above, as the entire module assembly heats up, there is a potential manufacturing problem: the solder connections which hold the semiconductor chips to the ceramic substrate could reach a sufficiently high temperature to melt. Thermal fatigue is a particular problem when the module is reworked several times.